Because islands and the Temperate Zone are hotspots of non-native species invasions, biologists have hypothesized that mainlands and tropics are harder to invade. The latter regions have more species, and the idea is that biodiversity confers “biotic resistance.” But new research overturns this theory for birds, showing that invasion success depends on factors such as the new environment’s suitability to the introduced species.

“Within biogeographic regions, species-rich locations are as easy to invade as species-poor locations,” say Tim Blackburn of the University of Birmingham, United Kingdom, and Richard Duncan of Lincoln University in Canterbury, New Zealand, in the November 8, 2001 issue of Nature.

To test the biotic resistance theory, Blackburn and Duncan assessed the outcomes of all known bird introductions worldwide. The researchers determined whether invasion success correlated with a variety of factors, including latitude of introduction, island vs. mainland, biogeographic region, and success of related species.

Blackburn and Duncan found that the biotic resistance theory does not apply to birds. For one thing, the ease of invasibility was not greater for islands or temperate regions. Moreover, two of the most species-rich biogeographic regions (the Afrotropics and Central/South America) were among the easiest to invade.

“The traditional perception, that species-poor islands and temperate locations are easy to invade, probably reflects the greater total number of avian introductions to these locations,” say Blackburn and Duncan.

Blackburn and Duncan found that a number of other factors did correlate with invasion success, notably a species’ geographical range size and whether the locations of origin and introduction were within the same biogeographical region or at similar latitudes. It makes sense that species with large ranges are more likely to invade for two reasons, say the researchers. First, such species are more likely to flourish under a greater variety of environmental conditions, and second, they are more available for capture and transport and so tend to have been introduced more often and in larger numbers.

Likewise, it makes sense that invasions are more successful within a biogeographical region or when the latitudes of origin and introduction are similar. Places in the same region or at similar latitudes are more likely to have similar climates and habitat features. “Introduction success is enhanced if species are matched with suitable environments,” say Blackburn and Duncan.

In sum, the researchers conclude that the success of bird invasions cannot be predicted by general factors (such as the latitude of introduction or the success of relatives) but rather depends on specific factors (such as environmental suitability and range size). “This may help explain why general features of invaders have been hard to characterize but shows that an understanding of introduction success is possible nonetheless,” say Blackburn and Duncan.